The minimum energy required to remove an electron from a given atom or molecule to infinity. Originally the ionization potential was defined as the minimum potential through which an electron would fall in order to ionize the atom, and was measured in volts.

The electron usually removed from the atom or molecule is in the outermost orbit, i.e. the least strongly bound electron. Some atoms and molecules may be ionized by the removal of an electron that is not in the outermost orbit, i.e. one that is more strongly bound. The resulting ion will be in an excited state. The ionization potential for removal of the least strongly bound electron is designated the first ionization potential (I₁) and results in an ion in the ground state. Ionization by the removal of the second (or subsequent) least strongly bound electron requires a greater ionization potential, termed the second (or third, etc.) ionization potential (I₂, I₃, etc.).